The present invention relates to a semiconductor device, and more particularly to an integrated circuit device including a lateral transistor and a diode connected between the base and emitter electrodes of the transistor.
One of the basic requirements for an integrated circuit formed in a single semiconductor substrate is the elimination of unwanted leakage current arising among plural semiconductor elements, each comprising the integrated circuit.
The effect of this leakage current is explained on an example of prior-art integrated circuit construction. In FIG. 1 of the drawings reference numerals 1 and 2 designate a bipolar transistor and a diode, respectively. The bipolar transistor 1 is provided with an n-type semiconductor substrate 3, a p-type impurity region 4 formed by doping p-type impurities in the surface portion of the semiconductor substrate 3, and another p-type impurity region 5 formed by doping p-type impurities in the surface portion of the semiconductor substrate 3 which is separated by a predetermined distance from the above-mentioned impurity region 4. The impurity region 4 serves as a collector, and the impurity region 5 as an emitter, respectively.
The diode 2, on the other hand, is formed in the surface portion of the semiconductor substrate 3 distant from the bipolar transistor 1, and comprises an impurity region 6 formed by doping p-type impurites and another impurity region 7 formed by doping n-type impurities within the surface portion of the impurity region 6. The impurity regions 6 and 7 serve as an anode and a cathode of the diode 2, respectively. The surface of the semiconductor substrate 3 is covered with a silicon dioxide film 8 in which contact holes 9, 10, 11 and 12 are formed, the contact holes 9, 10, 11 and 12 being located above the impurity regions 4, 5, 6 and 7, respectively. An aluminum collector electrode 13 is in contact with the impurity region 4 through the contact hole 9. Likewise, an aluminum emitter electrode 14 is in contact with the impurity region 5 through the contact hole 10 and also with the impurity region 7 through the contact hole 12. On the other hand the impurity region 6 is in contact with a connecting electrode 15 through the contact hole 11. This connecting electrode 15 is further connected to a high concentration n-type region 16 formed in the surface portion of the substrate 3, the high concentration n-type region enhancing ohmic contact between the connecting electrode 15 and the semiconductor substrate 3.
The bipolar transistor 1 and the diode 2 connected as described above form parts of a protection circuit which prevents a thyristor 21 shown in FIG. 2 from unwanted turning on. When a specified voltage VA is applied to the P gate of the thyristor 21 in ordinary usage, this thyristor 21 is turned on and a large current flows into the thyristor 21. If an unwanted surge voltage is applied to this thyristor 21, however, it tends to be turned on forcedly. In order to prevent the thyristor 21 from making such misoperation, the bipolar transistor 1 functions to supply a current from the N-gate electrode of the thyristor 21 to its N-base electrode by the agency of the collector capacity of the bipolar transistor 1.
The diode 2 is provided to maintain the above mentioned function of the bipolar transistor 1 by discharging to the N-gate the charges accumulated in the collector capacity of the bipolar transistor 1 when voltage fluctuation due to surge voltage is repeated. If the thyristor 21 is turned on despite of the functions of the bipolar transistor 1 and the diode 2, another bipolar transistor 23 functions to short-circuit the section between P gate and the cathode of the thyristor 21 to turn it off.
However, if a leakage current flows from the impurity region 4 to the impurity region 6, the above described function of the bipolar transistor 1 is impaired and the protection circuit 22 with the above-mentioned features fails to function correctly. For a PNP type bipolar transistor, the cause of this leakage current lies in the fact that an inversion layer is generated in the surface portion of the semiconductor substrate 3 between the impurity regions 4 and 6 due to presence of a negative voltage with respect to the semiconductor substrate 3 applied to the collector electrode 13. However, if the inversion layer reaches to the surface portion of the semiconductor substrate 3 below the emitter electrode 14, the inversion layer is extinguished by the agency of the positive voltage applied to the emitter electrode 14. In the prior-art integrated circuit the impurity region 6 is not fully covered with the emitter electrode 14 so that there is no effect of the emitter electrode 14 on the exposed surface portion of the semiconductor substratre 3. As a result the above-mentioned inversion layer can finaly attain the impurity region 6 passing through the surface portion between the emitter electrode 14 and the connecting electrode 15. This inversion layer serves as a current path, through which the leakage current is generated.
The goal of the present invention is the fabrication of an integrated circuit which is free from the leakage current due to the inversion layer generated by the potential of the electrode 13.